The long term goals of this proposal is to identify the function of anti-Fab antibodies (AlphaFABA), especially in relation to the pathogenesis of rheumatoid arthritis (RA) and the modulation of immune responses. We have recently determined that a large portion of serum anti-Fab antibodies (AlphaFABA) in RA demonstrate restricted clonality, circulate as immune complexes, and are comprised of minor IgG subclasses, notably IgG4. Due to these unique properties it is unlikely that they have been previously investigated in detail. The experiments in this proposal are designed to: 1) identify differences in the clonal and subclass expression of AlphaFABA in the RA and normal population and determine if the increased levels of AlphaFABA in RA are the result of the expression of additional antibody clones or the result of amplification of normal existing clones; 2) identify the heterogeneity of the Fab antigenic repetoire which bind AlphaFAB; 3) determine whether the clontypic and subclass expression of AlphaFABA is a dynamic process and how it is influenced by transient immune responses, and 4) establish whether differential expression of AlphaFABA in RA patients and normal individuals is the result of V-region gene specific or IgG subclass gene specific switching mechanisms. The clonality of AlphaFABA will be determined by isoelectric focusing techniques (IEF). The distribution and quantity of different AlphaFABA IgG subclasses will be determined by preparatively isolating AlphaFABA by chromatofocusing and using enzyme-linked immunosorbent assays (ELISA) to identify their IgG subclasses. The heterogeneity of the alphaFABA antigenic repetoire will be determined by affinity purification of different AlphaFABA subclasses and determining their reactivity with various F(ab prime)2 molecules in ELISA. The dynamics of clonotypic and subclass expression of AlphaFABA will be examined by IEF and ELISA assays of patients' sera obtained at various times in the course of their disease. Finally, the degree of AlphaFABA idiotype cross-reactivity among different AlphaFABA subclasses will be investigated using monoclonal anti-AlphaFAB antibodies generated by hybridomas in order to establish whether the expression of different AlphaFABA is the result of V-region gene or IgG subclass gene switching.